(a) Field of the Invention
The present invention relates to a camera exposure controlling method camera and a diaphragm driving device for controlling the camera exposure and, more particularly, to a diaphragm driving device that prevents an overrun defect, employing the aperture-priority method.
(b) Description of the Related Art
Generally, in order to determine the amount of camera exposure, a method of controlling F-numbers, which indicate the relative size of the lens aperture opening (for example, 1.4, 2.8, 4, 5.6, 8, 11, 16, 22, etc.), or a method of controlling the shutter speed (which typically has values of B, 1, 2, 4, 1/30, 1/60, 1/125, 1/250, 1/500, 1/1000, etc.), can be used.
Among the methods, the former is called "aperture priority method" while the latter called the "shutter speed priority method".
When a camera has a diaphragm and a shutter, as a separate item, either the aperture priority method or the shutter priority method can be implemented without any difficulty depending on the situation. However, when a diaphragm is used for a shutter as well as for controlling the aperture, the aperture and the shutter speed are controlled at the same time. In the latter case, the sector may be formed as a single type or a double type.
In the single type sector, as shown in FIG. 7, a diaphragm ring 200 for controlling the aperture has a cam 202 contacting a sector opening and closing device 204. Because this cam 202 has a smooth surface to enable continuous movement, the single type sector tends to have overexposure problems, shown as R in FIG. 6. Although the rotating diaphragm ring 200 stops at a predetermined position on the cam 202, inertia may force the diaphragm ring to overrun after the diaphragm ring stops at the right position with a diminishing oscillation. Because of this overrun exposure, the total amount of light reaching the film plane cannot be uniformly controlled.
Therefore, an aperture priority method cannot be employed in such a single type sector.
A double type sector has a first set of sectors that is used to control the aperture and a second set of sectors that is used to control the shutter speed. Therefore, while a double type sector may prevent the overrun exposure problem, it requires a complicated mechanism and an expensive complex structure.